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Performance of Selected Metalized Coatings and Sealers

Performance of Selected Metalized Coatings and Sealerson Lock and Dam Facilitiesby Tim Race, Vince Hock, and Al Beitelman,US Army Construction Engineering Research LaboratoryPrior to World War II, the US Army Corps of Engineers had established an extensive navigation system along the Ohio River. The system consisted of 53 lock and dam facilities that allowed navigation from Pittsburgh downriver to the confluence of the Ohio and Mississippi Rivers, near Cairo, IL. Beginning in the 1950's, the original system was augmented by the construction of 18 modern locks and dams; only 2 of the old facilities are still in operation. The newer dams traverse a total river elevation of just over 400 ft, averaging about 22 ft/dam. The pool elevation change of the new dams is much greater than that of the old dams, which averaged 8 ft.

The characteristics of the new dams generated design constraints that have led to maintenance problems. For example, water flowing under the tainter gates of the new dams may attain velocities of over 35 ft/sec. In addition, large chunks of debris, such as logs several feet in diameter, commonly flow downriver (Fig.1)[all figures available upon request]. To alleviate undertow and turbulence downstream from the new dams, submerged baffles were located downstream from each tainter gate. The baffles create a hydraulic effect against the downstream tainter skinplates.

The high water velocities coupled with river debris and suspended particles such as river sand result in a highly abrasive environment. Standard Corps of Engineers vinyl coatings formulated for locks and dams rapidly erode in this environment. Coating failure and substrate corrosion typically occur in 1 to 2 years.

A further indication of the severity of abrasion on Ohio River lock and dam facilities is the longevity of the Corps' vinyl paint systems on Mississippi River facilities. Abrasion on the Mississippi River results mainly from ice flow during the winter.

The Corps' vinyls were first used along the Mississippi in 1950. Applied in 1950, a red lead vinyl primer, with vinyl intermediate and topcoat system, was not reapplied at Lock and Dam 23 at Hannibal, MO until 1981. The interior of the gate, which experienced no abrasion, was inspected and returned to service at that time without repainting.

Abrasion at Greenup Locks and Dam on the Ohio River is estimated to be about 15 times as severe as the abrasion at a typical Mississippi River dam. This difference is due mainly to the particular design features of Ohio and Mississippi River dams.

To address the maintenance problems encountered on the lock and dam structures in the Ohio River, the Corps of Engineers undertook an evaluation of the performance of several metal coatings and sealers.

This article outlines the criteria used to select coatings and sealers for evaluation, explains application procedures for each material, describes the field evaluation of the materials used, and discuss the results of the evaluation procedure.

Selection of Coatings and Sealing Systems for Evaluation

Eight metalized coating and sealer systems were selected for evaluation.

The rationale for the selection of each coating and sealer system is given below.

Metallic Coating Selection

Metallic coatings were selected for application at Belleville Locks and Dam near Parkersburg, WV. Metals were selected for their hardness, degree of adhesion, commercial availability, and corrosion resistance.

The aluminum-bronze alloy selected has a nominal composition of 10 percent aluminum, 89 percent copper, and 1 percent iron. Aluminum-bronze containing 5-12 percent aluminum has excellent corrosion resistance and has been used for applications requiring abrasion resistance. Aluminum is added to copper to stabilize the corrosion product as a barrier film. The compositional requirements of this material are described in Mil-W-6712C, Wire, Metalizing.

Stainless steels are well known for their corrosion resistance to fresh water, salt water, and chemical solutions; 18-8 stainless steel was selected for application. Austenitic stainless steel have demonstrated excellent resistance to impingement attack in a seawater jet. Because of its excellent resistance to cavitation erosion, 18-8 has been used on sonar domes. Stainless steel wires are described in Mil-W-6712C.

Zinc, which has a long history as a metalizing material, was also selected for application at Belleville. Zinc coatings were flamed sprayed on bridges as early as the 1920s. The Corps of Engineers first evaluated zinc metalizing on the Mississippi River Dam No. 15 in 1939.

Zinc is not as hard as metallic coatings such as stainless steel, Monel, and aluminum-bronze. However, zinc is sufficiently anodic to steel to be sacrificial. Stainless steel, Monel, and aluminum-bronze are cathodic to mild steel. Zinc readily forms a protective passive barrier film composed primarily of zinc carbonate.

It has been suggested that alloy of 85-15 zinc-aluminum coating is superior to either pure zinc or aluminum coatings. Adding aluminum to zinc raises the density of the applied material and assists in forming a protective barrier oxide. Zinc-aluminum alloys are not as hard as stainless steel, aluminum-bronze, and copper-nickel coatings, but 85-15 zinc- aluminum coatings will sacrificially protect mild steel and will act as a protective barrier.

Sealing of Metallic Coatings

Organic sealer used in conjunction with metalized coatings form a synergistic system that is reported to outperform the sum of its parts. This is especially true of anti-corrosive metalizing systems for marine and industrial atmospheres.

Because of the porosity of metalized coatings, organic sealers are needed to protect the coating and substrate from the environment. The density of sprayed metals is typically 85 percent to 95 percent of the density of the wire from which it was sprayed. The total oxide included in the sprayed film accounts for only 0.5 percent to 3 percent of the reduction; the density reduction is due primarily to the characteristic porosity of sprayed metal coatings.

Thicker metalized films (typically in excess of 10 mils) should be less prone to substrate corrosion due to moisture migration through pores. Metalized coatings of zinc and aluminum in the range of 5 to 9 mils are reported to be sufficient to prevent access to the substrate through continuous pores. Metals such as zinc, aluminum, and zinc-aluminum alloys may have reduced porosities due to the formation of protective oxides upon weathering.

Extensive experimentation and field testing with sealers have been performed. Many sealers have been evaluated, including epoxies, alkyds, urethanes, phenolics, and vinyls.

In general, the research showed that the selected sealer system should be appropriate for the intended exposure independent of the metalizing. For example, marine coatings are appropriate for sealing metalized coatings in marine environments. Copper, aluminum, zinc, and stainless steel coatings may require the use of surface pretreatment to improve adhesion of the sealer. Initial sealer coats should be applied at low viscosities and should flow readily to penetrate the porous metalized surface. Sealer topcoats may be applied at normal viscosities.

The most important criterion for sealers applied at Belleville Locks and Dam was that the coating sealer system be compatible with the intended exposure, namely, fresh water immersion. Selected sealers also had to exhibit a high degree of adhesion to the metallic coating and tolerate enough thinning to flow readily and fill the pores of the metalized coating. The development of a Corps of Engineers, military, industry, or other standard specification describing the sealer material was also considered to be desirable.

Tainter Gate Number 5 of Belleville Locks and Dam was metalized in part during the summers of 1986 and 1987.

Each metallic coating was applied to approximately 700 sq. ft. of Gate Number 5. Aluminum-bronze alloy and 18-8 stainless steel were applied and sealed in 1986; zinc and 85-15 zinc-aluminum were applied and sealed in 1987. Metalizing extended across the entire 110-foot length of the gate, from just above the downstream water line to the bottom of the gate and 3 ft. up the upstream skinplate.

The entire area to be metalized was given an initial blast cleaning with Ottawa silica sand to remove any remaining vinyl coating. All areas to be metalized were reblasted with aluminum oxide abrasive, not more than 4 hours prior to metalizing. A White Metal grade in accordance with SSPC-SP 5 was achieved. The surface profile was specified to be in the 2- to 4-mil (50- to 100-micron) range and was verified using replica tape. Prior to metalizing, the abrasive blasted substrate was cleaned by blowndown with clean, dry, compressed air.

The contractor was required to submit field samples of metalized coatings and sealers for laboratory evaluation. Metallic coatings were applied in accordance with contract specifications and evaluated for thickness and qualitative adhesion. Sealer systems were then applied to the panels per contract specifications.

Coating thickness were monitored during the application in accordance with SSPC-PA 2 using a dry film thickness gage and a plastic shim. The plastic shim was used to help average the surface roughness and to provide more accurate results. Magnetic thickness gages could not be used on the 18-8 stainless steel, so in this case, the field sample served as the sole reference and standard for coating application.

Adhesion was qualitatively evaluated by cutting through the coating using a knife of chidel. If after cutting, the coating or any part of it 1/2 inch square of larger could be lifted from the substrate without actually cutting the metal away, the adhesion was deemed unsatisfactory.

Application of Aluminum-Bronze Coating

The aluminum-bronze coating was field-applied using a two-wire arc spray gun.

Material was applied by blocking out a four-square-foot area on the substrate and making parallel spray passes with 20 percent to 50 percent overlap per pass. Successive coats needed to achieve the specified 10- to 15-mil thickness were applied at 90 degrees to the previous coat. Surface-to-gun distance was maintained at approximately 6 in., and gun speed was maintained at 3 to 6 in./sec. Production rates with no downtime were estimated at 60 to 80 sq. ft./hr. Three spray passes were needed to achieve the desired coating thickness. A 0.0625-inch to 0.125-inch spark gap was maintained between the 2 wires. Voltage was kept at 28 to 32 volts.

Figure 2 (all figures available upon request) shows the application of aluminum-bronze by the two-wire arc at Belleville. The two-wire arc method produced a bright green light, necessitating the use of dark filters such as those that welders employ. The process also produced a high noise level. Observers and workers wore ear protection at all times while the two-wire arc gun was in use. The arc process also produced a significant quantity of smoke and metal fumes. The smoke caused considerable staining of the areas adjacent to the metalizing gun, including the aluminum-bronze itself.

Coating thickness was measured with various gages. An Elecomet Inspector Gage and a Positector 2000 yielded similar results. In addition to the magnetic thickness gages, a micrometer was used to measure the thickness of the coating applied to a one-by three-inch coupon taped to the skinplate. The thickness of the coupon was subtracted from the micrometer reading to obtain the coating thickness. This method consistently gave measurements 1 to 2 mils higher than the magnetic gages. Actual measurement on the applied coating produced reading of 7 to 22 mils, with an average of 10 to 15 mils.

Wire-feed problems were a major drawback in the application of the aluminum-bronze; downtime exceeded work time. The contractor suggested that the wire had been coiled improperly, causing twisting in the feeder tubes.

Noise levels were high enough to cause discomfort even with the use of ear plugs. One observer stationed in the work area also reported temporary flu-like symptoms characteristic of the metal fume fever.

Application of Stainless Steel Coating

Application of the stainless steel coating proceeded rapidly after initial wire-feed problems of the arc gun were cleared up. However, inclement weather caused considerable downtime, delaying completion of other metalizing sections until 1987.

Thickness measurements were determined employing the on-by three-inch coupons as previously described. Coating thickness averaged 10 to 15 mils. General observations on sight and sound were similar to those made for application of aluminum-bronze.

Application of 85-15 Zinc-Aluminum Coating

Zinc aluminum was applied using a wire flame spray gun. The spray speed using acetylene gas with 0.125-inch diameter wire was 25 lb./hr. Higher deposit rates may be achieved with gun modifications. Relative gas flows were 45 oxygen at 30 psi, 42 acetylene at 15 psi, and 53 air at 70 psi. Flow rates were measured with a flowmeter.

Wire flame spray with 85-15 zinc-aluminum was much less noisy than the two-wire arc process. In addition, the light produced by the process could be observed directly without discomfort, and the process created much less smoke and fewer metal fumes than the electric arc spray. No occurrence of the metal fume fever was noted during the application of 85-15.

The large amounts of wire consumed during application necessitated a regular maintenance schedule for the gun. The contractor dismantled, cleaned, and lubricated the gun, and replaced worn parts as needed.

Coating thickness was specified at 10 to 15 mils. Measured valued ranged from 12 to 25 mils. The average measured thickness was about 16 mils. Production rates averaged from to 50 to 60 sq. ft./hr.

Application of Zinc Coating

Zinc was also applied using a wire flame gun. The spray speed using an oxyacetylene flame was 32 lb./hr. Relative gas flows were similar to those used for 85-15 zinc-aluminum. Again, 0.125-inch wire was used.

Noise, smoke, fumes, and flame brightness were the same as for zinc-aluminum flame spray.

Coating thickness was specified at 10 to 15 mils. Measured values ranged from 12 to 24 mils with and average thickness of 17 mils. Production rates again ranged from 50 to 60 sq. ft./hr.

Field Evaluation of Metalized Coatings and Sealers

Results of the field evaluation are given below and are summarized in Table 1.

Aluminum-Bronze

The sealed aluminum-bronze section has been inspected twice since being applied.

Figure 3A (all figures available upon request) shows a portion of the aluminum-bronze coating after 9 months of exposure on the downstream side of the tainter gate; the 3 sealer systems are clearly delineated on the figure. Dark areas indicate the presence of corrosion products on the surface. One noteworthy defect is the long, horizontal, corroded area in the three-coat, V-766E-sealed section. Presumably, this failure was caused by a large piece of debris, such as a tree, cutting through the sealer and some or all of the aluminum-bronze coating.

Pinpoint rusting can also be seen in each of the different sealed areas. Most of the corrosion occurred along the downstream skinplate waterline where the abrasion is most severe. In one small area (about 9 sq. in.) along the waterline of the downstream skinplate, the aluminum- bronze had delaminated, exposing the substrate. The bay areas, formed by stiffeners at the bottom of the gate, showed very little corrosion. The small area metalized with aluminum- bronze on the upstream skinplate was also in good condition, with only light pinpoint rusting and no scratches.

Figure 3B (all figures available upon request) shows that the delaminated area described above increased after 20 months. The bays and upstream skinplate were in better overall condition after 20 months than the radius and upper portions of metalizing.

Large areas of sealer had worn off in some areas, particularly along the waterline of the downstream skinplate. The wash primer with vinyl topcoats exhibited areas of white and gray, evidence that the gray topcoat had eroded to expose the white intermediate coat. Portions of the metalized surface exhibited greater pinpoint rusting than did adjacent metalized areas. Thickness measurements in these areas indicated that they often corresponded to areas of relatively thin metalizing.

Stainless Steel

The stainless steel section was also inspected after 9 and 20 months of service.

Figures 3C and 3D (all figures available upon request) show the 3 sealer systems applied to the 18-8 stainless steel after 9 months of exposure. There were no deep cuts in the 18-8, nor was there any delamination of the coating as was observed with the aluminum-bronze coating.

There was, however, considerable pinpoint corrosion for each sealer system in the areas shown in the figures. The bays at the bottom of the gate were in better condition that the upper portions of the downstream skinplate; they exhibited only slight pinpoint rushing. The upstream skinplate portion of the stainless metalizing had not been seriously scarred or eroded. Each of the sealer systems and adjacent vinyl-coated surface had formed blisters.

After 20 months of exposure, about the same amount of surface rust was visible. Considerable algal growth and a black stain were visible at 20 months. The bays at the bottom of the gate remained in fair condition, with only moderate pinpoint corrosion. The paint sealers generally eroded from the waterline of the downstream skinplate. Each sealer system exhibited a fairly uniform appearance. There was slightly less corrosion visible on the section sealed with 2 coats of Mil-P-24441.

Zinc and Zinc-Aluminum

When the zinc and zinc-aluminum coating were examined in June 1988, both had been in test for 10 months at a time. Aluminum-bronze and stainless steel were in test for 20 months. The zinc-aluminum coating supported more algal growth than the other metalized coatings. The entire downstream skinplate waterline section was bright green from growth. No red rust was visible on the zinc-aluminum-coated section, and there was no measurable loss of coating thickness. The coating surface was somewhat polished along the waterline of the downstream skinplate, indicating that some wear was occurring.

Both aluminum-bronze and 18-8 stainless steel were applied by the two-wire arc process in 1986. These coatings exhibited signs of failure after only 9 months of exposure. Failure consisted of pinpoint rusting for both coatings. Cutting and delamination of the aluminum- bronze also occurred.

Aluminum-bronze and stainless steel coating are cathodic, too, and will corrode the mild steel substrate when immersed in fresh water. It was hoped that the sealers selected would prevent water from reaching the substrate/metalized coating interface. This was not the case, as evidenced by the galvanic corrosion present.

The aluminum-bronze showed some delamination along the curved radius of the downstream skinplate. This area probably experienced the most abrasion. Delamination may have resulted from poor bond strength caused by surface contamination from moisture, flash rusting, or oil. Insufficient preheating of the substrate also may have caused poor mechanical bonding of the coating.

Poor blasting that resulted in a deficient surface profile could have also caused poor adhesion. A high surface profile helps to dissipate the high shear stress that forms as sprayed coatings cool. Substrate corrosion may also have played a role in the coating delamination, since corrosion products expand to the greater volume they require.

Corrosion products were also evident along several cuts in the aluminum-bronze. No cutting of the stainless steel coating occurred. Presumably, the only factor at issue here would be the relative hardness of the coatings.

Blistering due to galvanic corrosion was evident along the upstream skinplate in areas adjacent to the stainless steel metalizing. A similar effect has been noted in vinyl-coated areas adjacent to cable trays and protective cladding where stainless steel is often used on dam gates.

Sealers for Aluminum-Bronze and 18-8 Stainless Steel

Sealers were used in conjunction with aluminum-bronze and stainless steel metalizing primarily to prevent galvanic corrosion of the substrate due to water migration through the porus metallic coating. Researchers had indicated that minimum coating thickness of 5 to 9 mils would prevent penetration to the mild steel substrate through continuous pores.

Pinpoint rusting was noted for each sealer system used with aluminum-bronze and 18-8 stainless steel coatings. Clearly the sealers did not perform as intended. The pinpoint corrosion is obviously the result of moisture penetration and galvanic corrosion. Pinholes and holidays in the applied sealers may have allowed water migration, causing corrosion. When immersed, all organic coatings will eventually be penetrated by moisture. A wicking action at continuous pores may have provided enough moisture for corrosion to begin. Finally, erosion of the metallic coatings themselves could have exposed continuous porosity, allowing corrosion to occur.

One sealer system was marginally better on the aluminum-bronze, and another was better on stainless steel. On aluminum-bronze, the wash primer system with vinyl intermediate and topcoats outperformed the epoxy system and the V-766E system. This is probably due to the improved adhesion of coatings applied over wash primers on certain substrate. Also, the wash primer may penetrate the porous metalizing to a greater extent.

The epoxy sealer system was slightly better than the other 2 sealer systems applied to 18-8 stainless steel coating. The mode of failure for the stainless steel coating was primarily pinpoint rusting. Evidently, the epoxy system reduced the migration of moisture by either being more resistant to erosion of by having a lower water permeability.

Zinc and 85-15 Zinc-Aluminum

Zinc and 85-15 zinc-aluminum metallic coatings were selected for application at Belleville after the initial inspection of the aluminum-bronze and 18-8 stainless steel coatings revealed premature failure. The comparative hardness of metals such as Monel and stainless steel was sacrificed in favor of metals that would galvanically protect the mild steel substrate.

After 10 months of exposure, zinc and zinc-aluminum coatings showed no signs of deterioration. The galvanic quality of zinc and 85-15 zinc-aluminum coatings in contact with mild steel should prevent corrosion from appearing as long as a continuous film of metal coating remains. The formation of zinc and aluminum oxides and hydrates on the surface and in the pores should reduce the coating porosity.

Zinc alloys containing 15 percent to 17 percent aluminum reportedly have improved corrosion resistance. Research indicated that alloys of this type posses the advantageous properties of both metals. Thermally sprayed zinc-aluminum alloys yield a two-phase coating. The zinc-rich phase affords galvanic protection of the steel substrate; the aluminum- rich phase passivates and has good barrier properties. However, these results may not be valid under all exposure conditions. Multi-phase alloys may form tiny corrosion because of potential differences between the alloy phases. This condition may worsen as the aluminum- rich phase passivates, which could further shift its potential relative to the zinc-rich phase. This may result in blistering of the zinc-aluminum coating because of the formation of aluminum oxide. Just how the Ohio River environment will affect the 85-15 zinc-aluminum alloy in the long run is not known at this time. Reportedly, 85-15 zinc-aluminum has a higher bond strength and is harder than zinc sprayed alone.

Sealing of Zinc and 85-15 Zinc-Aluminum

Only the vinyl butyral wash primer was used to seal the zinc and zinc-aluminum coatings. a probable two- to three-year extension in the service life of the zinc and 85-15 coatings could be expected from a fully developed sealer system with topcoats. The vinyl butyral wash primer was intended only to seal the porosity, not to protect the coating from environmental degradation and erosion.

Conclusion

Several conclusions may be drawn from the study of thermal spray metallic coatings applied at Belleville Locks and Dam.

Coatings anodic to mild steel, such as 85-15 zinc-aluminum and pure zinc, appear to offer an attractive alternative to conventional paint coatings for use in abrasion-corrosion environments such as those found at Belleville and other Ohio River Dam facilities.

Annual performance evaluation of the coatings applied at Belleville is necessary to establish the service life of zinc and 85-15 zinc-aluminum coatings. It is anticipated that these coatings will prove to be cost-effective. A Corps of Engineers civil works guide specification for the use of thermal-spray, metallic coatings on hydraulic structures is being prepared. This guide specification will address safety, surface preparation, materials, sealers, application, inspection, quality control, and recommended uses of thermal-spray, metallic coatings.

Coatings cathodic to mild steel, such as stainless steel and aluminum-bronze, should not be used in fresh water immersion, where exclusion of water to the substrate cannot be guaranteed. The presence of water at the mild steel substrate/metalized coating interface will cause galvanic corrosion. Seal coats did not adequately prevent the penetration of water to the substrate interface at Belleville.

About the Authors Tim Race has worked extensively with high performance coatings for locks and dams in his position as a Principal Investigator for the US Army Construction Engineering Research Laboratory (CERL). He joined CERL in 1981 after earning a B.S. in Chemistry in 1980. Race belongs to SSPC and ASM International. Vince Hook, also Principal Investigator for CERL, has worked on developing projects involving materials such as thermally sprayed, corrosion-resistant coatings for civil works applications; ion-plated ceramic anodes; and anti-scale corrosion coatings for potable water heat exchangers. He holds an M.S. in Metalurgy and a B.S. in Chemistry. Alfred D. Beitelman has been with the US Army Corps of Engineers since 1970 and is currently Director of the Paint Technology Center at CERL. He is an active and long- time SSPC member, a NACE-Certified Coatings Inspector, and a member of ASTM and AWWA. He has a B.A. in Chemistry. All of the authors can be reached at US Army, CERL, P.O. Box 4005, Champaign, IL 61824-4005.